Titanium dioxide (TiO2) is a widely used white pigment popular for its brightness and high refractive index. Titanium dioxide provides whiteness and opacity to products such as paints, coatings, plastics, papers, inks, cosmetics, foods, and medicines.
Titanium dioxide is a photoactive material and TiO2 particles with size range below 0.2 micron are capable of absorbing ultraviolet light. As a result, electrons can be energized, creating holes in the valence bands and excitons in the conduction bands. In pigment applications, it is important to reduce the photoactivity of titanium dioxide because it can induce undesired redox reactions that degrade paint or coating materials.
In pigment processing, titanium dioxide photoactivity can be reduced by surface treating or coating TiO2 particles with inorganic and/or electron harvesting materials. These coating materials can prevent ultraviolet light from reaching the surface of the TiO2 particles. Other materials that can be used are able to quickly harvest or stabilize the energized electrons in the conduction bands before they initiate a redox reaction. For example, oxides of different elements, such as silicon and zinc, as well as different organic chemicals, can be used for this purpose.
The surface treatment of TiO2 particles with oxide coating is a major processing step in titanium dioxide pigment production. It is a delicate and complex process where reaction conditions such as pH, ionic concentration, and temperature, have to be carefully monitored and adjusted. This is essential to ensure formation of the coating oxide material and to achieve excellent pigment performance. On a manufacturing plant scale, the complexity of the surface treatment process is magnified.
Traditionally, monitoring the pH has been needed not only to ensure that the coating oxide is formed, but also in most cases, to determine the type of the oxide forming. Generally, in surface treatment processes, pH control is carried out using strong acids, such as HCl and H2SO4, and strong alkalis, such as NaOH. Adjusting and controlling the pH, particularly at plant production scale, is a non-trivial step that consumes a large amount of time and chemicals. The use of acids and alkalis also increases the ionic concentration in the aqueous suspension of TiO2 particles. This can induce unfavorable steric effects in the suspension, causing particles to agglomerate or flocculate, and rendering the surface treatment process ineffective. In addition, a high ionic content can also affect subsequent pigment processing steps. For instance, the efficiency of washing and filtration steps can be affected by the extent of ionic content in the titanium dioxide suspension. The higher the ionic content, the more water is consumed and the more time is needed.
A need exists for a titanium dioxide surface treatment method that utilizes a low ionic content and/or eliminates the use of acids or alkalis to adjust the pH. The method should desirably have a shortened surface treatment process time and provide enhanced efficiency of subsequent washing and filtration steps. The method should also desirably result in effective coating of TiO2 particles for use as a pigment.